Add TWI Peripheral driver for XMEGA
TWI Master has a sub-structure
Fix NON-NULL attributes, add for TWI_t.
Minor bugfixes / missed compile stuff.
Bugfixes with the protocol.

 - Bitmasks need to be equal, not just result in non-zero
 - Only read status once in case it changes under us
 - Logical and not binary and when checking for acks on sending
 - We need to stop the bus when receiving the last byte
 - Force the bus into idle mode when we are Initing
Merge branch 'xmega-twi' of github.com:jamuraa/lufa into xmega-twi-integration
Fix XMEGA TWI driver indentation and style to conform to the rest of the LUFA codebase.
Add XMEGA TWI to the ModuleTest build test.
Add XMEGA TWI peripheral driver to the Atmel Studio extension XML.
Use seperate ATTR_NON_NULL_PTR_ARG() decorators in the XMEGA TWI driver to prevent Doxygen from choking.